1. Field of the Invention
The present invention relates to a process for forming a prestress anchorage by drawing a steel sleeve over an elongate metal core having substantially round section.
2. Description of the Prior Art
In the technique for manufacturing prestressed concrete part, the concrete part to be prestressed is subjected to compression by jacks which bear against the concrete and which pull one or more metal bars or more metal cables passing through one or more sheaths buried in the concrete, the metal bars or the metal cables being then locked by anchorage against the concrete. In some known techniques for forming anchorages (see for example patents FR Nos. 1241 157 and 1530 660 and the patent U.S. Pat. No. 3,559,270,) a metal sleeve is anchored at least at one end of each cable or each metal bar by being drawn through a draw-plate, so as to form an anchorage head adapted to abut against a bearing plate, this latter itself bearing against the concrete in the case of a dead anchorage or being possibly connected to the pulling jack in the case of an active or mobile anchorage. It is evident that the anchorage between the metal sleeve and the metal cable or bar must be able to withstand the intense pulling forces to which the cable or the bar is subjected in use, in general several tens of tons-force (a ton-force is equivalent to 0.98.10.sup.3 daN), even more than 100 tons-force, without the metal sleeve sliding or becoming detached from the metal bar or cable. Among the known processes for forming such an anchorage by drawing a steel sleeve over a bar or a metal cable, a sleeve is generally used having an inner diameter greater than the the outer diameter of the metal bar or cable on which the sleeve is to be anchored, and between the sleeve and the cable or the bar is interposed a metal insert, grooved on the inside and made rough on the outside by indenting or by milling, namely a helical spring with jointing turns made from tempered steel as is stated in the two above mentioned French patents. In both cases, the metal insert or the helical spring is made from a material which has a hardness greater than that of the sleeve (the hardness of the sleeve having to be relatively low so as to allow it to be drawn through a draw-plate,) and greater than that of the metal bar or cable. Thus, during drawing of the sleeve, the metal insert or helical spring becomes incrusted in the outer surface of the metal bar or cable and provides anchorage between the sleeve and the bar or cable. However, to form an anchorage of the above described type, having a given breakage strength, it was necessary up to present to use a sleeve and a metal insert or helical spring having a relatively large length. Following tests carried out by the applicant, this may be explained in the following way. In so far as the anchorage are concerned having a metal insert, this latter reduces the effective volume of the material of the sleeve which participates in the anchorage, so that this reduction in volume must be compensated for by an increase in the length of the sleeve. In so far as anchorages are concerned having the helical spring, it has been discovered that the turns of the spring tend to slant or to lie down during the operation of drawing the sleeve and the spring is impressed or incrusted more or less evenly in the outer surface of the metal bar or cable, which reduces the efficiency of the anchorage.
Another process is known for butting together steel bars for reinforced concrete, which consists in using a sleeve which is threaded or toothed on the inside and whose inner toothed or threaded surface is hardened, the sleeve being then fixed on the two steel bars to be butted together by drawing a sleeve and the bars through a matrix (see patent CH No. 532.702).
The steel bars usually used for reinforced concrete generally have a breakage strength of 40 kg/mm.sup.2, i.e. a much lower strength than that of the bars or the stranded cables used for prestress anchorages, which generally have a breakage strength of 180 kg/mm.sup.2. In fact, in reinforced concrete, the bars are only subjected to tensile or compression forces due to the weight of the concrete work or to the load supported by this latter. On the other hand, as was pointed out above, in prestress anchorages, the bars or cables are permanently subjected to a very high tension, in general several tens of tons-force, even more than a 100 tons-force.
Moreover, it will be noted that in the patent CH No. 532.702, the inner teeth of the sleeve are surface hardened by application of a flame or case hardened. The thickness of the layer hardened by such a process is relatively small and would be quite inefficient for forming a prestress anchorage.